Glass manufacturing systems are commonly used to form various glass products such as LCD sheet glass. It is known to manufacture sheet glass by downwardly flowing molten glass over a forming wedge and drawing a glass ribbon from the root of the forming wedge. Pairs of downstream rolls are frequently provided to provide pulling forces to facilitate drawing of the glass ribbon from the forming device.
Downstream rolls are used in the manufacture of sheet glass. Downstream rolls are preferably designed to contact the glass ribbon at its outer edges, specifically, in regions just inboard of the thickened beads which exist at the very edges of the ribbon. Some examples of downstream rolls are pulling rolls, which are used to apply tension to the ribbon of glass from which the sheets are formed and thus control nominal sheet thickness.
A downstream roll needs to meet a number of criteria. First, the roll needs to be able to withstand the high temperatures associated with newly formed glass for extended periods of time. Second, the downstream roll must not give off excessive amounts of particles, which can adhere to the glass and form surface defects (known as “onclusions”). For glass that is to be used in such demanding applications as substrates for flat panel displays, onclusions must be kept to very low levels. Third, if a pulling roll, the downstream roll must be able to produce enough force to control the thickness of the glass ribbon. In order not to damage the central portion of the ribbon that becomes the usable finished glass, the roll can only contact the ribbon over a limited area at its edges. Thus, the required pulling forces must be generated using only this area while imparting minimal point loading stress. However, the forces cannot be too large since this can create surface damage which can propagate into the usable central portion of the ribbon or cause cracking of the glass ribbon. When cracking occurs, a great number of glass particles are generated which can damage and contaminate the roll materials. Therefore, the roll must also be hard enough to allow continuous operation while simultaneously soft enough to minimize the stresses resulting from cast off glass particles. Ideally, the roll is soft enough to allow these cast off particles to be driven into the roll in order to maintain the same contact stress on the ribbon.
The existing downstream rolls have not been able to fully satisfy these competing criteria of long life, controlled force application, low contamination and reduced penetration resistance. The present invention addresses these shortcomings in the art and provides improved downstream rolls that achieve higher levels of performance than existing downstream rolls.